Generally, there is a limit to a frequency resource in radio communication. Therefore, it is necessary to have a technology that enhances effective use of a frequency. To effectively use the frequency, various studies have been made regarding a technology that enhances a transmission speed per 1 Hz such as multilevel modulation and an error correction code, a cellular system using a method of frequency reuse per cell, a CDMA communication system using one cell reuse, an interference canceller, and the like.
Meanwhile, in recent years, a concept of “cognitive radio” has attracted a lot of attention, in which a free frequency band, a radio wave of which has not been used, is used to perform radio communication. Such a cognitive radio method has a mechanism in which a radio base station itself searches for a currently free frequency band around the radio base station from among all of the frequency bands to be used, and uses the frequency for communication. For example, when there is a frequency band that is not currently used in each radio system according to a period of time and land area, it becomes possible to use this frequency band in the radio base station in the radio communication. In this way, according to the cognitive radio method, the effective use of a frequency can be expected.
The radio base station estimates a frequency band, a radio wave of which has not been used, by a radio terminal that exists in its own radio base station or in a coverage area covered by its own radio base station and acquires information of a frequency band that has not been used by a primary system, and performs the communication using the frequency band. At this time, it is required to use a free frequency band not to deteriorate receiving quality of other radio system by interfering the other radio systems to which a targeted frequency band is allocated or preferential use of the targeted frequency band is allowed.
For example, when a radio wave is transmitted by predetermined transmission power, it is necessary to use the radio wave at a place sufficiently away from other radio system. Also, for example, when the radio wave is used at a place not sufficiently away from other radio system, it is necessary to keep the transmission power low.
In such a technology using the frequency band, the radio wave of which has not been used, Non Patent Literature 1 discloses, for example, a study and an evaluation result of a case where the free frequency is used by all base stations with the same transmission power. To be more specific, it is assumed to have terrestrial digital broadcasting as a primary system of high priority and a wireless regional area networks (WRAN) as a secondary system of low priority. At this time, Non Patent Literature 1 discloses a result of evaluating frequency use efficiency and coverage efficiency of a case using a model in which a plurality of base stations of a secondary system uses a free frequency band of the primary system and a distance between the secondary base stations and a range of evaluation areas are used as parameters.
Also, in relation to a transmission power control method in a radio base station, for example, Patent Literature 1 discloses a method of determining transmission power of the radio base station based on an interference amount in the radio base station.